Generally, there are two methods for producing element boron in industry.
(1) Magnesium reduction process, in which boric acid and magnesium powder are mainly taken as raw materials, the boric acid for industrial use is put in a stainless steel plate and then the stainless steel plate is put in a tube furnace to evenly heat to a temperature of 250 DEG C under a pressure-reducing condition, so that the boric acid dehydrates to form boron oxide: 2H3BO3═B2O3+3H2O; then the boron oxide is crushed to 80 mesh and is fully mixed with the magnesium powder at the ratio of 3:1 (mass ratio); and then the mixture is put in a reaction tube to perform a reduction reaction at a temperature of between 850 and 900 DEG C in the tube furnace under vacuum, so that the boron oxide is reduced to the element boron, wherein this reaction is a thermal reaction which can be finished quickly: B2O3+3Mg=3MgO+2B; the materials obtained after the reaction is finished are dipped in water for two days and then are boiled for 4 hours in hydrochloric acid so as to be free of impurities such as magnesium oxide, then the materials are washed off acid by water; in order to remove the impurities, it is necessary to repeat acid pickling and water washing for one time in the same condition, thus, boron powder with boron content of about 85% is obtained; in order to improve the quality of boron and to further remove magnesium, the boron powder above can be added to the boron oxide which is ten times the mass of the boron powder to be evenly mixed and heated to a temperature of between 800 and 850 DEG C in the reaction furnace under vacuum and kept for 3 to 4 hours; then the material is taken out and washed off boron oxide by water; after processes of acid pickling and water washing again, the material is filtered and dried, thus, boron powder with boron content of over 90% is obtained.
(2) Aluminium reduction process, in which industrial borax is generally taken as a raw material and is put in a melting furnace of over 750 DEG C at normal pressure to dehydrate 10 crystal water to form anhydride sodium tetraborate; after being cooled, coarse-crushed and fine-crushed, the anhydride sodium tetraborate is well mixed with sulphur and aluminium powder at a certain ratio, then the mixture is put in a cast-iron reaction furnace to perform a reaction at a high temperature: Na2B4O7+4Al=4B+Na2Al2O4+Al2O3−; after being cooled, frits are taken out of the reaction furnace and are crushed first, then the crushed frits are dipped in hydrochloric acid and then in hydrofluoric acid, next, the crushed frits are washed by water and alkali (5 mass percent NaOH solution), finally, the crushed frits are wasted by water, separated and dried to obtain element boron.
The two methods above mainly have disadvantages of low yield rate and high preparation cost, and the content of the product obtained is less than 90%.
The method for preparing sodium fluoroaluminate (sodium cryolite) in industry generally adopts a synthesis method, in which anhydrous hydrofluoric acid reacts with aluminium hydroxide to form fluoaluminic acid; then the fluoaluminic acid reacts with sodium hydroxide at a high temperature; after processes of filtering, drying, melting and crushing, the sodium fluoroaluminate is prepared, wherein the reaction formula is as follows: 6HF+Al(OH)3=AlF3.3HF+3H2O, AlF3.3HF+3KOH=Na3AlF6+3H2O; the sodium cryolite synthesized by this method has a relative molecular weight of 209.94, with a molecular formula of AlF3.mNaF (m=3.0) and a melting point of 1000 DEG C; the sodium cryolite synthesized by the industrial synthesis method generally has a molecular ratio of m=2.0-3.0, and it is difficult to obtain the relatively pure sodium cryolite of a low molecular weight with a molecular ratio of m=1.0-1.5.